1. Field of the Invention
The present invention relates to a radio-wave propagation characteristic forecasting system and its method, particularly to a radio-wave propagation characteristic forecasting method using a geometric-optical method using the so-called ray launching.
2. Description of the Prior Art
A radio-wave propagation simulator is used to support the arrangement of base stations or host systems in a wireless communication system. Reception power and delay spread are evaluated at an optional reception point by a radio-wave propagation simulator to decide a setting place of an appropriate transmitting station and as a result, the reduction of the number of base stations to be arranged is efficiently executed.
The radio-wave propagation simulation is roughly divided into the simulation using a statistical method and the simulation using a deterministic method. The statistical method is a method for deciding a parameter of an estimate equation using a distance or frequency as an argument through the multivariate analysis in accordance with a lot of data obtained from actual measurement of a propagation loss. The deterministic method is a method for obtaining a propagation loss or delay value by assuming radio waves radiated from an antenna as an aggregate of radio-wave rays and synthesizing rays geometrically-optically propagating while repeating reflection and transmission and reaching an observation point.
The geometric-optical method is further divided into the imaging method and the ray launching method. The imaging method is a method for deciding a ray reflection/transmission route connecting a transmission point with a reception point by obtaining a imaging point against a reflection surface. A reflection/transmission route can be uniquely obtained when a transmission point, reception point, and reflection/transmission object are defined. Therefore, the imaging method is a strict method for searching a ray propagation route. However, the ray launching method is a method for regarding a ray passing the vicinity of a reception point as a ray reaching the reception point by radiating rays from an antenna in a certain direction independently of a reception point and obtaining a ray propagation route according to reflection and transmission, which is disclosed in Japanese Patent Laid-Open No. 9-33584.
FIG. 15 is an illustration for explaining operations of the ray launching method when an observation region 100, transmission point A18, reception point A20, and two objects A06 and A09 are provided. In FIG. 15, operations are explained by restricting the operations to a two-dimensional plane for simplification. In fact, however, operations may be performed in a three-dimensional space.
First, a ray A03 is radiated from the transmission point A18 in the direction of a certain propagation route. It is examined for all objects in the observation region whether the ray radiated in the direction collides with the objects present in the observation region 100. The ray A03 collides with the object A06 at a reflection point A19 and as a result, a transmitting ray A07 and a reflected ray A11 are generated. The ray 11 generated due to reflection further collides with the object A09, so that a transmitting ray A15 and a reflected ray A10 are generated. The reflected ray A10 passes the vicinity of the reception point A20, the ray is handled as an incoming wave at an observation point.
Specifically, the reception intensity and the incoming delay time defined in accordance with the total of propagation distances of the rays A03, A11, and A10 are recorded as shown in FIG. 16. The abscissa axis 101 of FIG. 16 shows the delay time required for the rays to achieve the total of the above propagation distances from the transmission point A18 up to the observation point A20 and the ordinate axis 102 shows reception intensities of the rays passing through the route of the total of the propagation distances.
The reflection/transmission ray search same as the case of the above propagation route is repeated for the ray A03 radiated in the direction of the propagation route from the transmission point A18 also on transmitted waves A07 to A15. When a ray passes the vicinity of the reception point A20, it is handled as a coming wave the same as the case of the ray A10 and the above processing is continued until a search end condition is satisfied. The search end condition is set to a condition when the reception electric-field intensity at a reflection/transmission point becomes lower than a predetermined value.
After the reflection/transmission route search of a ray radiated in the direction of the ray A03 from the transmission point A18 is completed, the same launching is executed by changing emission angles of the ray to be radiated from the transmission point A18 like the case of, for example, the ray A21 in another propagation route and examined on all emission directions of the transmission point A18 or some of emission directions previously defined. Finally, FIG. 17 is obtained which is a delay profile for the reception point A20. The abscissa axis 201 of FIG. 17 shows the delay time until a ray comes from the transmission point A18 and the ordinate axis 202 shows the reception intensity of a ray passing through the route. Reception power is obtained by the sum of reception intensities of all paths and a delay spread showing the degree of a distortion is given by the standard deviation of delay times.
The above ray launching method is not a method for strictly obtaining the solution of the propagation route of a ray connecting a transmission point with a reception point like the imaging method but it is a method for approximately providing the solution. Therefore, it has a feature capable of shortening the time required for propagation route search.
A ray spread corresponding to the propagation distance from the transmission point is defined for each ray shown in FIG. 15. The ray spread is a spread region defined in the vicinity of a ray and the spread is defined so that it increases as the distance from the transmission point increases and decreases as the number of rays radiated from the transmission point increases.
In FIG. 18, the same portion as that in FIG. 15 is shown by the same symbol. FIG. 18 specifically shows the envelope of a ray spread, in which the ray spread envelope for the ray A03 is defined by A01 and A02, that for the ray A11 is defined by A12 and A16, and that for the ray A10 is defined by A22 and A23. This ray spread is used to decide whether to regard a ray passing the vicinity of the reception point A20 as a ray reaching the reception point.
Specifically, when the reception point A20 is given, the distance D between the point A20 and the ray A10 is compared with the spread radius S of the ray spread at the spot concerned. When S is equal to or larger than D, the ray is handled as an arrival ray at the observation, and the delay time and reception intensity at the reception point A20 are recorded by considering the propagation distance from a transmission point, reflection number, distance D, and etc.
The delay profile characteristic at the reception point is obtained by applying the above described operation to all rays radiated from the transmission point A18, recording delay times and reception intensities of arrival rays incorporated into the reception point one by one, and synthesizing the delay times and reception intensities.
Main geometric-optical components in radio wave propagation are a reflected wave and a transmitted wave. However, to more accurately estimate a radio-wave propagation characteristic, it is necessary to consider a diffracted wave which is a nongeometric-optical component. In this case, diffraction is defined as a phenomenon that when a ray collides with an edge of a structure, a radio wave propagates while curving in a direction other than the traveling direction of the ray. Though a diffracted wave is originally not a geometric-optical component, the UTD method (Uniform Theory of Diffraction) described in (“A uniform geometrical theory of diffraction for an edge in a perfectly conducting surface,” Proc. IEEE, vol. 62, pp. 1448-1461, 1974) is generally used as a method for handling a diffraction phenomenon in a geometric-optical range. The UTD method models a diffractive wave by generating a plurality of rays having a radiation angle equal to the incident angle to a diffraction edge around the diffraction edge.
FIG. 19 is a schematic view for specifically explaining the UTD method. A case is assumed in which a ray R001 radiated from a transmission point S001 enters a diffraction edge E002 of a structure (object) E001. In the case of the UTD method, the diffraction edge R002 is radiated from a diffraction point D001 so that the angle Ti formed between the diffraction edge E002 and the incident ray R001 becomes equal to the angle To formed between the diffraction edge E002 and a diffracted ray R002. In FIG. 19, only one diffracted ray is shown. However, innumerable diffracted rays satisfying the above condition are generated along the side face of a cone C001 set to the outside of the structure E001.
In the ray launching method, the route tracing of a ray is performed by radiating rays from a transmission point at predetermined intervals and detecting the collision between the rays and a structure. However, because a ray is a line defined in a three-dimensional space having an infinitesimal size, the possibility that the ray collides with an edge of the structure provided as a line is also very low. Therefore, there is a problem that a diffraction phenomenon cannot correctly be estimated because diffracted waves are hardly generated only by simply applying the conventional UTD method to the ray launching method.